Abstract

Given the limited intermolecular spaces available in dense liquids, the large amplitudes of highly excited, low frequency vibrational modes pose an interesting dilemma for large molecules in solution. We carry out molecular dynamics calculations of the lowest frequency (“warping”) mode of perylene dissolved in liquid argon, and demonstrate that vibrational excitation of this mode should cause identifiable changes in local solvation shell structure. But while the same kinds of solvent structural rearrangements can cause the non-equilibrium relaxation dynamics of highly excited diatomic rotors in liquids to differ substantially from equilibrium dynamics, our simulations also indicate that the non-equilibrium vibrational energy relaxation of large-amplitude vibrational overtones in liquids should show no such deviations from linear response. This observation seems to be a generic feature of large-moment-arm vibrational degrees of freedom and is therefore probably not specific to our choice of model system: The lowest frequency (largest amplitude) cases probably dissipate energy too quickly and the higher frequency (more slowly relaxing) cases most likely have solvent displacements too small to generate significant nonlinearities in simple nonpolar solvents. Vibrational kinetic energy relaxation, in particular, seems to be especially and surprisingly linear.

Received 02 May 2012Accepted 20 June 2012Published online 12 July 2012

Acknowledgments:

This work was supported by the U.S. National Science Foundation (NSF) (Grant No. CHE-0809385). We are grateful to Mark Maroncelli for extensive discussions concerning the vibrational dynamics and spectroscopy of the low-frequency modes of perylene dissolved in liquids. We are also pleased to thank Guohua Tao and Crystal Nguyen for helpful discussions about the design and implementation of the calculations reported here.